[UPNP/JSONRPC] add source setting allowSharing to secure source access from UPNP/WebServer

[UPNP/JSONRPC] add source setting allowSharing to secure source access from UPNP/WebServer

Continuation of 1220d8c
Add a source flag allowsharing defaulting to true to allow or disallow
access to a source from UPNP/WebServer

The checks are now available as a function in FileUtils and common to UPNP and JSON
Also make the check correctly works in JSON as current implementation have flow allowing browsing any directory with Files.GetDirectory.